PSD image texture - material not updating?
Image used as textures will not update automatically every time the file is changed, but you don't need to close blender to make the changes visible.

The easiest way to do this is to re-load the image in the UV/Image editor window. The shortcut is AltR



On the Properties panel of images used as texture there is also a button to reload the image from disk.



There is an addon to sychronize gimp with blender, but I don't know if it still works. I avoid using GIMP... but the info is in this answer: 

Can Krita and Blender be linked?